Image-forming elements which provide images by selective removal of image-forming layers are known. Generally, image formation in such elements results from selective exposure of the image-forming layer(s) to actinic radiation and subsequent development. Such elements are known as radiation sensitive elements and include lithographic plates, photoresists, relief plates, color-proofing elements and color filter arrays.
Radiation sensitive compositions useful in such elements are generally categorized as being either positive-working or negative-working. Positive-working compositions are used to form radiation sensitive coatings which can be selectively solubilized in a suitable solvent in radiation-struck areas, thus leaving behind a positive image. In a common form, positive-working compositions can be coated from solvents and developed using aqueous solutions.
Negative-working compositions can be prepared from polymers which crosslink and become insolubilized in radiation-struck areas. Such compositions are ordinarily coated from organic solvents. A developer, which can be aqueous or organic solvent based depending upon the nature of the radiation sensitive compositions, is used to remove the unexposed portions of the coating to form a negative image.
In many image-forming elements, e.g. lithographic plates and photoresists, it is desirable to have either a visible indication of exposure or a visible image before development. This is convenient in step and repeat exposure operations where the elements are stored for some time between exposure and development, or where it is desirable to evaluate the image before development. To provide this visibility, it is frequently the practice to include in the image-forming composition a dye which is capable of either printout or bleachout on exposure of the coating prepared from the composition. A wide variety of indicator dyes useful for this purpose are known, as described for example in U.S. Pat. No. 3,920,457 (issued Nov. 18, 1975 to Cunningham et al).
In other instances, it is desirable to have a visible indication of the image after development. This can be useful, for instance, in what are known as "proofing" or inspection operations or after final development of lithographic plates or photoresists. The dyes described above are not usually suitable for providing image visibility after development because they are frequently partially or totally removed during development. Hence, workers in the art typically use suitable pigments to provide a readily visible imaage after exposure and development.
The pigments used in image-forming elements to provide such post-development image visibility are colorants of very small particle size which usually exist in very small agglomerates. When dispersed in a composition comprising suitable solvents, binders and addenda commonly used to formulate radiation sensitive compositions, these small agglomerates are attracted to each other and clump together and form larger agglomerates. When these agglomerates become large enough, they settle out of the dispersion over time. If not broken up or redispersed, such agglomerates can plug coating equipment and cause streaks, nonuniform pigment density and other defects in the coatings and otherwise ruin the resulting product. Conventional milling techniques are generally inadequate to break up the agglomerates to harmless sizes or to keep the pigment dispersed. Hence, workers in the art have resorted to expensive and time-consuming redispersion techniques that keep pigments dispersed until they are mixed with radiation sensitive materials and coated.
This dispersion problem hs been mitigated to a limited degree by use of dispersing agents, e.g. commercially-available surfactants, such as charged and non-charged fluorocarbons, phosphate esters, condensed naphthalene sulfonates, dialkyl sodium sulfosuccinates; zinc, copper, iron and calcium naphthenates; calcium ligno sulfonates; and fatty acids.
U.S. Pat. No. 3,929,489 (issued Dec. 30, 1975 to Arcesi et al) and U.S. Pat. No. 3,920,457 (issued Nov. 18, 1975 to Cunningham et al) describe certain radiation sensitive compositions useful in the preparation of lithographic plates. These compositions comprise particular photocrosslinkable polyesters having anionic moieties. Generally, pigments are added to these compositions while dispersed in suitable solvents. The dispersing agents described hereinabove, and particularly the fluorocarbon surfactants, are used to disperse the pigments and minimize pigment agglomeration.
However, it has been evident that the dispersing agents fail to reduce pigment agglomeration in the dispersions to an acceptable extent. When mixed with the radiation sensitive compositions, the resulting coating compositions have unacceptably-large pigment agglomerates, poor coatability and considerable pigment density variations. The coating compositions cannot be stored for long periods of time without becoming increasingly unstable, meaning clumps of pigment settle out. Yet, such pigments cannot be redisersed in such compositions because the high shear needed for redispersion would undesirably crosslink the photocrosslinkable polyester.
Hence, there is a need in the art for stabilized particulate pigment dispersions useful in image-forming compositions and coatings which are substantially free of pigment agglomerates.